Why " how fun it is to.." incorrect?

I encountered a question that went like this:

_______ fun it is to watch a 4D movie! A. What B. What a C. How D. How a

The key to this question is A.

I've read an explanation which says " ‘How’ can only precede an adjective in this kind of sentence, e.g. “How happy I am,” or an adverb in a similar kind of sentence, e.g. “How well you dance.” Since ‘fun’ is a noun, only “What fun it is” is correct. It’s the same construction as “What nonsense you speak” and “What an awful road this is.” However, “How much fun it is” would be correct.

Of course, some people might say “How fun it is,” but it’s still grammatically incorrect."

I was confused, because "fun" is also an adjective word. Then why is "how fun" grammatically incorrect?

It used to be that fun was a noun but not an adjective. See Ngram.

If fun were just a noun, you could not say

How fun it is to ...

Let's replace fun with the noun joy to see what happens:

What joy it is to ...
*How joy it is to ...

The second sentence sounds completely wrong (and doesn't even appear in Ngrams).

Unfortunately, the makers of the test are around fifty years out of date; fun is now well-established as an adjective in English. And for adjectives, you can use them with how

how easy it is to ...
how strange it is to ...
how pleasant it is to ...
how simple it is to ...

See Ngrams.

Lexico dictionary still classifies the adjective fun as informal, and it appears that usage of how fun it is to is still lagging what fun it is to (See Ngrams), but I expect it will catch up in a few more decades.